Turbochargers deliver air, at greater density than would be possible in the normally aspirated configuration, to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. This can enable the use of a smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, thus reducing the mass and aerodynamic frontal area of the vehicle.
Turbochargers are a type of forced induction system which use the exhaust flow, entering the turbine housing from the engine exhaust manifold, to drive a turbine wheel (51), which is located in the turbine housing. The turbine wheel is solidly affixed to a shaft to become the shaft and wheel assembly, the other end of which assembly contains a compressor wheel (20) which is mounted to the stub shaft (56) end of the shaft and wheel and held in position by the clamp load from a compressor nut (29). The primary function of the turbine wheel is providing rotational power to drive the compressor.
The compressor stage consists of a wheel (20) and it's housing. Filtered air is drawn axially into the inlet of the compressor cover by the rotation of the compressor wheel. The power generated by the turbine stage to the shaft and wheel drives the compressor wheel to produce a combination of static pressure with some residual kinetic energy and heat. The pressurized gas exits the compressor cover through the compressor discharge and is delivered, usually via an intercooler, to the engine intake.
In one aspect of compressor stage performance, the efficiency of the compressor stage is influenced by the clearances between the compressor wheel contour (28) and the matching contour in the compressor cover. The closer the compressor wheel contour is to the compressor cover contour, the higher the efficiency of the stage. The closer the wheel is to the cover, the higher the chance of a compressor wheel rub; so there has to exist a compromise between improving efficiency and improving durability.
The wheel in the compressor stage of a typical turbocharger does not rotate about the geometric axis of the turbocharger, but rather describes orbits roughly about the geometric center of the turbocharger. The geometric center referred to here is the geometric axis (100) FIG. 1, of the turbocharger.
The dynamic excursions taken by the shaft are attributed to a number of factors including: the unbalance of the rotating assembly, the excitation of the pedestal (i.e., the engine and exhaust manifold), and the low speed excitation from the vehicle's interface with the ground.
The net effect of these excursions taken by the wheels is that the design of the typical turbocharger has clearances far greater than those desired for aerodynamic efficiency levels.
The typical turbocharger is fed with oil from the engine. This oil, at a pressure, typically equal to that of the engine, performs several functions. The oil is delivered to both sides of the journal bearings (30), via oil galleries (82 and 83) to provide a double hydrodynamic squeeze film, the pressures of which exert reactionary forces of the shaft on the ID of the bearing and of the OD of the bearing on the bearing housing bore. The oil films provide attenuation of the reactionary forces to reduce the amplitude of the excursions of the shaft. The oil also functions to remove heat from the turbocharger as the oil drains through the oil drain (85) to the crankcase of the engine.
A typical turbocharger design has two adjacent bearing systems: one on the compressor-end of the bearing housing, and one on the turbine-end of the bearing housing. Each system has two interfaces: the interface of the rotating shaft on the I.D. of the floating bearing, and the interface of the O.D. of the floating bearing on the fixed bore of the bearing housing.
The stiffness and damping capacities of the typical turbocharger double hydrodynamic squeeze film bearings are a compromise between the thickness of the film generated by the rotational speed of the bearing elements, the clearance between said elements, and the oil flow limitations due to the propensity of turbochargers to pass oil through the piston ring seals at either end of the shaft.
The use of REBs in a turbocharger solves several problems, including: high oil flow rates, bearing damping, and power losses through the bearing system.
FIG. 1 depicts a typical turbocharger double hydrodynamic squeeze film bearings configuration. In this configuration, pressure fed oil is received by the bearing housing (3) though an oil inlet (80) from the engine. The oil is pressure fed through the oil galleries (82 and 83) to the bearing housing and journal bearing bore (4). For both the turbine-end and compressor-end bearings, the oil flow is delivered to the shaft and wheel journal bearing zones at which points the oil is distributed around the shaft to generate an oil film between the shaft surface (52) and the inner bore of the floating journal bearings (30). On the outside of the journal bearings (30), a like oil film is generated by the rotation of the journal bearing against the bearing housing journal bearing bore (4).
In the typical turbocharger depicted in FIG. 1, the thrust bearing (19) is also a hydrodynamic or fluid film type of bearing. In this configuration, the stationary thrust bearing is fed oil from the oil gallery (81) to feed a ramp and pad design of bearing. The oil is driven into a wedge shape by the relative motion of the thrust washer (40) and the thrust washer area of the flinger (44), which is mounted to the shaft, against the static thrust ramp and pad. This bearing controls the axial position of the rotating assembly.
For the typical 76 mm turbine wheel-sized turbocharger discussed above, the oil flow is in the region of 4200 to 6200 grams per minute.
One method of increasing the efficiency of the turbocharger has been the adoption of ball bearings to support the rotating assembly. There are several improvements that come with the adoption of rolling element bearing turbochargers. There is an improvement in transient response due to the reduction in power losses, especially at low turbocharger RPM, of the REB system over the typical turbocharger bearing system. The power losses in REB systems are less than those for typical sleeve type turbocharger bearing systems, and improved transient response (both being critical aspects of engine-out emissions). REB systems can support much greater thrust loads than can typical turbocharger bearing systems making the thrust component more robust. Since typical ramp and pad thrust bearings require a large percentage of the oil flow delivered to the turbocharger, and REB systems require less oil flow (than a typical turbocharger bearing system), then less oil flow is required for a REB system with the positive consequence that there is less propensity for oil passage to the compressor or turbine stages where that oil can poison the catalyst.
While ball bearing systems provide these efficiency and transient performance gains, the damping capacity of ball bearings is not as good as that of the typical turbocharger double hydrodynamic squeeze film bearings, so the ball bearings are retained in a steel cartridge, which is suspended within the bearing housing by an oil film between the O.D. of the cartridge and the I.D. of the bearing housing bore. The oil is used for damping of shaft critical events and for lubrication of the bearings.
U.S. Pat. No. 5,145,334 (Gutknecht) and U.S. Pat. No. 7,214,037 (Mavrosakis) teach methods for the retention of said bearing cartridge in the bearing housing. These methods allow for a floating bearing cartridge for which the axial and rotational forces are reacted upon by a post secured in the bearing housing while allowing for otherwise unconstrained motion of the bearing cartridge in the bearing housing.
For a turbocharger bearing system, achieving the required speed is a critical factor. Achieving that speed with an acceptable life for the system is the next most important factor. In any turbocharger, for meeting emissions requirements in the engine to which the turbocharger is applied, reducing the oil flow to the turbocharger and, by leakage, out of the turbocharger, is a major factor. Oil passage from the turbocharger through the piston ring seals is a major factor in the soluble organic fractions (SOF) component of particulate matter emissions. The only methods for reducing the oil passage through turbocharger piston ring seals is to either control the pressure differential across the seals (so that air does not flow from the turbocharger bearing housing to either the compressor or to the turbine stages), to control the barrier to this oil laden airflow through the seals, or to limit the quantity of oil in the turbocharger. The latter method is very effective and is one method which makes the use of REB turbochargers popular since the mean oil flow through the typical REB turbocharger example above is approximately 1400 grams/minute. By contrast, the mean oil flow through the same size sleeve bearing turbocharger is 3000 grams/minute or 218% more.
In high speed ball bearing development, much work has been done to improve the speed and life of the bearing system, especially in the area of the relationship between bearing power losses and oil flow. In NASA 2001-210462, the writers teach that: conventional jet lubrication fails to adequately cool and lubricate the inner race contact because lubricant is thrown centrifugally outward, and while increasing the flow rate results in carrying away more heat, it also adds to the heat generated from oil churning. The writers also note that bearing power loss is a direct function of oil flow to the bearing. Bearing life is an inverse function of temperature, the difference in temperature between the individual bearing ring components, and the resultant elastohydrodynamic film thickness.
FIG. 5 depicts the flow of heat from the turbine housing, through the bearing housing, to the bearings. The exhaust flow from the engine flows into the turbine housing foot and through the volute. The flow (110) is directed by the volute at the turbine wheel (51) where most of the heat energy is transformed into torque to drive the turbine wheel. The remaining heat energy is left in the exhaust flow (111) out of the turbine housing, and in the turbine wheel and the turbine housing material. Some of the heat energy flows up the shaft, where it encounters the air dam (58), which acts as a thermal block to some of the heat flow up the shaft. Some of the heat energy from the turbine housing is conducted through the joint between the turbine housing and the bearing housing. Some of the heat energy is radiated to the atmosphere.
The heat flow from the turbine housing to the bearing housing (3) is transmitted by conduction to the bearing cartridge of a REB turbocharger. In sleeve bearing turbochargers, the lubricating oil for the bearings carries a great deal of this heat out of the turbocharger to the engine oil heat rejection system. In REB turbochargers, since the oil flow is less than half that of a sleeve bearing turbocharger, the capability of heat rejection by the oil is far less.
With maximum speed and reduced oil flow as given requirements, that leaves only temperature as a variable in the turbocharger lifetime calculation.
Temperature limitations on ball bearings are expressed as the incoming temperature to the bearing races, the heat energy generated by the bearing races as a function of the work done by the bearing, and the heat rejection from the bearing assembly. In the case of a turbocharger, there is heat input to the inner race through the shaft and wheel and heat input from the bearing housing to the cartridge and to the outer race. The output of heat from the turbocharger is via the lubricating oil to the lower part of the bearing housing, and then to the engine oil drain system, where it becomes part of the vehicle heat rejection system.
One expensive and relatively complicated method for control of heat input to the bearing system is the use of water-cooled bearing systems. These bearing housings use the engine coolant to draw heat from the bearing housing to the engine coolant system. The castings for these water-cooled bearing housings are multi-cored, so as to leave voids through which the engine coolant flows to pull heat from the exhaust flow. This makes the castings expensive since the number of cores is much higher than those for air-cooled turbine housings. The complexity of the multiple coring add time and cost to the production of the casting, and because they draw heat out of the turbine housing, via the bearing housing, the total heat energy available to the turbine wheel is reduced. Water-cooled bearing housings do, however, provide one very important function, and that is that they prevent heat soak problems after engine shut down when the heat from the engine, exhaust manifold, and turbine housing flows back into the turbocharger. These heat soak problems can force the bearings into metal to metal contact, with the potential for yield in the components, through the result of different coefficients of expansion and tolerances of the internal components.
So it can be seen that the temperature of the inner and outer ball races is critical to bearing life. The heat soak problems for turbocharger REBs are exacerbated over those for sleeve type bearings. There is always a need with REB turbochargers to achieve the desired speed and life at the lowest cost.